1. Field of the Invention
The invention relates to transparent, colorless containers for liquids containing strong solvents such as acetone, for example nail polish bottles; and more particularly to a method for making such a bottle from a plastic material, and a bottle produced by the method.
The manufacture of containers for liquids such as nail polish and other materials used in the cosmetics industry involves the production of countless millions of bottles per year. Especially when they are used for cosmetics sold at retail, the appearance of the container is vital to the commercial success of the product. As a result the performance requirements for the container may be hard to meet, and the cost of the container may represent a significant part of the total manufacturing cost of the product.
In choosing container materials and methods of manufacturing, the weight, precision of manufacture, and ruggedness of the container are all critical to the impact on total selling price because of their impact on the cost of other aspects of manufacturing and delivery. Clearly, if the weight of the container becomes great, the shipping costs will go up. This is especially true of the cost of shipping an empty container to a place where it will be filled; bottle weight can also represent a major fraction of the weight of the finished, filled product especially when only a small volume of liquid is contained in the package.
Because of the exclusive use of automatic filling machinery in large scale manufacturing, it is important that containers be molded or otherwise formed with fairly precise dimensions. The cost of a bottle filling machine, and the speed at which it can handle the bottles, is closely related to the dimensional repeatability and fragility of the bottles themselves. For this reason efforts have long been made to develop bottles which would be lighter and yet more rugged, with capability for precision molding.
Cosmetics such as nail polish pose particularly stringent requirements on the container manufacturer. Nail polish is sold to consumers who wish to be able to compare the color, as viewed while it is in the container, with articles of clothing or other cosmetics. The consumer wishes to be assured that the apparent color of the polish while in the bottle is the same as that of the polish when applied to the fingernails, so that critical color matches may be made without extensive trial and error. As a result any color tint in the container is very disadvantageous.
Nail polish is a particularly difficult material to package because it must be presented in a transparent bottle which is resistant to attack by powerful organic solvents conventionally included in the polish, such as acetone, butyl acetate, ethyl acetate, toluene or methylethyl ketone (MEK). Historically, the material of choice for nail polish bottles has always been glass. This material is not only fully resistant to attack by the usual organic solvents, but it is readily available with an excellent combination of transparency and freedom from color tint. Glass bottles, however, suffer the disadvantage that they tend to be fragile, heavy, and not well controlled dimensionally as they leave the mold.
The fragility or brittleness of ordinary glass is of course well known. Because of this characteristic to avoid breakage the wall thickness of a glass bottle for nail polish must be relatively great. No manufacturer can afford lightly to take the chance that his product will break in the consumer's purse, damaging or ruining both the purse and its contents. As a result, most nail polish bottles are quite heavy in comparison to the weight of the contents, and are relatively bulky also for the volume contained. These thick wall sections are more difficult to mold accurately, and as a result the breakage of bottles during automatic filling is relatively high even though the bottles are quite rugged.
2. Description of the Prior Art
In an attempt to avoid all of the above disadvantages of glass, manufacturers have long sought a plastic bottle for use with materials such as nail polish. However, none of these attempts have been truly successful. One reason is that nail polish commonly contains a strong organic solvent. Acetone, toluene, ethyl acetate and butyl acetate are some of the more powerful of the widely available organic solvents, and will attack a great many of the relatively low priced or readily available plastic materials that were known to have the transparency and freedom from color desired in a container for nail polish. For example, the methyl methacrylates have good optical properties but are not solvent resistant. The search therefore has been directed toward the general classes of materials known as barrier resins; that is, those materials demonstrating low permeability to gases such as oxygen, and resistance to chemical attack. However, until now no readily available barrier resin has had the requisite physical properties of transparency and freedom from such color tinting as the yellowish cast of many widely known resins.
One plastic material which has been utilized for a nail polish bottle is low molecular weight acrylonitrile resin. However, bottles of this material have been cloudy in appearance, such that they are translucent rather than transparent, and usually have a purplish cast. This seriously impairs the potential customer's ability to compare the polish color with other materials in the showroom.
Attempts to develop solvent-resistant blends which will exhibit the desired physical characteristics have been unsuccessful. For example, the addition of chlorinated polyethylene to a styrene-acrylonitrile copolymer (SAN) improved resistance to impact and the desired solvents, but when more than 2% chlorinated polyethylene was added in an attempt to attain sufficient solvent resistance, the bottles became cloudy. Similar results were obtained when elastomeric polyester resins such as DuPont Hytrel 5556 series were added to SAN; 5% addition provided inadequate solvent resistance, and at higher levels cloudiness occured.
Another group of materials which have been unsuccessfully tried include polyethylene terephthalate mixtures (PETG). In their pure form these resins exhibit good optical and mechanical properties, but were found to be inadequately resistant to typical strong nail polish solvents. Addition of up to 5% of a modified polyethylene ionomer sold under the tradename Surlyn anhanced the mold release characteristics, but provided too small an increase in solvent resistance. The use of a straight Surlyn ionomer produced a bottle which looked good initially; but upon prolonged exposure to nail polish at elevated temperatures simulating warehouse storage, for example one week at 49.degree. C. (120.degree. F.), the walls softened and caved inward, and there was a loss of gross bottle weight due to solvent permeation through the walls.
Attempts were made to mold a quality nail polish bottle starting with resin that would be adequately resistant to the solvents. PVC, various polyolefins and polyacetals are resistant to one or more of the solvents found in nail polish, but are nearly opaque or inadequately transparent. However, the acrylonitrile resins are known to be good barrier resins, and offered more promise because they are at least semitransparent. In particular, a low molecular weight rubber-modified copolymer of 75% acylonitrile and 25% methylacrylate resin, sold by Vistron Corp. under the tradename of Barex 210 I was tried. This material exhibited satisfory mechanical properties and is unaffected by the nail polish solvents encountered. However, the bottles were blue-purple in color, and tended to show yellow or brown "burn" spots. Extensive attempts to correct the blue tint were unsuccessful, because the addition of sufficient dye to correct the unwanted color caused unexceptable loss of transparency. It was observed that yellowing of the resin was a problem if it was subjected to prolonged heating or processing, but exercising tight control on the processing parameters merely prevented excessive ayellowing and did not cure the blue-purple cast or eliminate burns.